1. Field of the Invention
The present invention relates to an exposure control system for use in a photographic camera, and more particularly to an automatic exposure time control system capable of film exposure even when object brightness, i.e., the brightness of an object or scene to be photographed, is so high as to require a shutter speed faster than the maximum shutter speed the shortest exposure time that the camera shutter mechanism can provide. Additionally, the camera exposure control system according to the present invention may be used for controlling the duration of electronic flash firing.
It should be noted that the shutter speed is not determined only in accordance with the object brightness, but other exposure factors such as set diaphragm aperture and film sensitivity values contribute to the shutter speed determination. Thus, the shutter speed can be lowered by changing the diaphragm aperture or the film used. However, there are limits to the changes of such exposure factors. Film sensitivity is usually fixed. The maximum diaphragm aperture value is designed for each objective lens. Therefore, the following description of the out-of-range object brightness and excessive shutter speed is made with the assumption that other exposure factors such as diaphragm aperture and film sensitivity values are fixed.
2. Description of the Prior Art
In the case of a high object brightness that requires an out-of-range shutter speed, it often occurs that the shutter operates without exposing the film to scene light, due to shutter closure before the exposure opening or slit is formed. Even in such a case, when the shutter is cocked for another photograph, the unexposed film is advanced or wound-up for a length of one frame, being regarded as used, thus resulting in a waste of film. In such circumstances, it is desirable for the film to be exposed even with slight overexposure rather than not to be exposed at all.
Japanese Published patent application No. 52-41659 discloses an exposure control device which, for that purpose, enables film exposure even when the object brightness is so high as to require an out-of-range shutter speed. That exposure control device employs a circuit schematically shown in FIG. 1, wherein the output of light measuring circuit 1, including photodiode PD, is connected to the base of transistor Q1 with the collector thereof connected through resistor R1 to timer capacitor C1. Switching circuit 2 is designed to control electromagnet Mg for initiating the shutter closing operation after a time lapse that is a function of the voltage across serially connected capacitor C1 and resistor R1.
In ordinary exposure control circuitry, no resistor is provided in series with timer capacitor C1, and switches S1 and S2 are turned ON and OFF, respectively, upon opening of the shutter, thereby causing the collector current of transistor Q1, which is commensurate with the output of light measuring circuit 1, to start the charging of the timer capacitor. When the charged voltage of the capacitor reaches a predetermined level, the switching circuit is inverted to actuate electromagnet Mg, thereby initiating shutter closing operation.
In contrast thereto, the exposure control device described in the aforementioned Japanese Published application includes resistor R1 interposed between capacitor C1 and transistor Q1, and switches S1 and S2 thereof are operated with a slight delay from the shutter opening by means of a delay or retard mechanism (not shown). Because of the existence of resistor R1, the input voltage of switching circuit 2 is lower by the voltage drop caused by the current flowing through resistor R1 for charging capacitor C1, than the input voltage without resistor R1. Thus, the inversion of switching circuit 2 occurs earlier than when resistor R1 is not provided, thereby compensating for the delay in the operation of switches S1 and S2. If the object brightness is extremely high, the charging current for capacitor C1 and the voltage drop across resistor R1 are so high that switching circuit 2 is inverted substantially simultaneously with the operation of switches S1 and S2. However, as switches S1 and S2 are mechanically constructed to operate with a given delay time from the shutter opening, exposure for a time corresponding to that delay is at least assured. Thus, the proposed prior art device ensures exposure at excessive object brightness without causing substantial photographic error under normal conditions.
If the prior art proposed exposure control circuit shown in FIG. 1 is also used for an automatically light controlled electronic flash, the amount of flash light controlled thereby tends to be smaller than a suitable value for the following reason. When the flash firing duration is controlled by the circuitry shown in FIG. 1, flash firing occurs in synchronization with the full opening of the shutter, and the reflected object light is converted by light measuring circuit 1 and transistor Q1 into an electric signal in the form of current which is integrated by capacitor C1. When the amount of light received by photocell PD reaches a predetermined level with the charged capacitor 1 voltage reaching a given value, switching circuit 2 is inverted to interrupt flash firing. In that case, since the voltage drop across resistor R1 rises at extremely rapid rate upon flash firing due to the current increase caused by the high intensity flash light reflected from an object, the input voltage of switching circuit 2 as a sum of the voltage drop and the terminal voltage of capacitor C1 reaches the predetermined level to stop the flash firing immediately after the commencement of the flash firing, resulting in the lack of the illumination of the object.